There is no over-the-counter “Plan B” pill for dogs, and human emergency contraception should never be given to them. But veterinarians do have several effective options for preventing or terminating pregnancy after an accidental mating. The approach depends on how much time has passed since the mating occurred and whether you want your dog to be able to breed in the future.
Why Human Plan B Won’t Work
Human emergency contraception contains a synthetic hormone called levonorgestrel, which works by delaying ovulation in women. Dogs reproduce through a completely different hormonal process, so the drug simply doesn’t have the same effect. Beyond being ineffective, giving your dog human medication introduces unnecessary risks. Even standard birth control pills, which contain low hormone concentrations, can cause vomiting, diarrhea, and loss of appetite in dogs. At higher doses, estrogen can suppress bone marrow function, reducing the production of red blood cells, white blood cells, and platelets. The packaging itself can also cause an intestinal blockage, which is sometimes a bigger concern than the hormones inside.
What Veterinarians Actually Use
Vets have several drug options for ending a canine pregnancy, and they’re chosen based on timing, the dog’s health, and what’s available in your country.
Progesterone Blockers
The closest thing to a “morning-after” option for dogs is a progesterone-blocking injection available in many countries outside the United States (sold under the brand name Alizin). It can be administered up to 45 days after mating and works by blocking the hormone that maintains pregnancy. Field trials show it is highly effective, with partial abortion occurring in only about 5% of cases and outright failure being extremely rare. This option preserves future fertility.
Prostaglandin Injections
Prostaglandins work by breaking down the structures in the ovaries that produce progesterone, the sole source of pregnancy-sustaining hormone in dogs. Without progesterone, the pregnancy cannot continue. Prostaglandins also cause the uterus to contract, helping expel its contents. The treatment requires multiple injections over several days, typically every 8 to 12 hours, and the dog needs to be monitored with ultrasound to confirm all fetal tissue has been evacuated. That means hospitalization or frequent vet visits, which adds to the cost.
Side effects are generally mild and short-lived: panting, trembling, nausea, and diarrhea. A newer synthetic version called cloprostenol targets the uterus more precisely and causes fewer of these systemic effects, making it the preferred choice at many practices.
Combination Protocols
Some vets combine a prostaglandin with a dopamine-stimulating drug called cabergoline, which suppresses the hormone that keeps progesterone production going. This combination has been reported to terminate pregnancy reliably with minimal side effects and is used more commonly in European countries where cabergoline is licensed as a veterinary drug. An oral corticosteroid protocol lasting about 9.5 days is another option, with reported efficacy around 97%. Pregnancy typically ends 7 to 13 days after starting treatment.
Why the “Mismate Shot” Fell Out of Favor
Years ago, vets commonly gave estrogen injections shortly after an accidental mating. This practice has been largely abandoned. Estrogen-based mismate shots carry serious risks, including life-threatening bone marrow suppression and a dangerous uterine infection called pyometra. The response to estrogen varies unpredictably between dogs: the same dose that causes only mild effects in one dog can be fatal in another. Veterinary guidelines now state that safe and effective doses of estrogen for this purpose simply do not exist.
Emergency Spay as a Permanent Option
If you don’t plan to breed your dog in the future, an emergency spay (ovariohysterectomy) is the most definitive solution. It can be performed at any stage of pregnancy and guarantees termination. It also eliminates the possibility of future accidental pregnancies, heat cycles, and reduces the long-term risk of reproductive cancers and uterine infections.
There’s a tradeoff, though. A routine spay is scheduled when the uterus is small and blood supply to it is low, making surgery safer and recovery smoother. Spaying during pregnancy means operating on a larger, more blood-rich uterus, which increases complication risks. Recovery takes one to two weeks, with vet check-ups typically scheduled around days 3 and 10 after surgery. An emergency spay becomes an especially practical choice when the pregnancy is already past day 45, since the drug options become less effective or are no longer recommended that late.
What to Do Right After an Accidental Mating
Time matters. Contact your vet as soon as possible after the mating occurs. The more options you’ll have, and the simpler those options tend to be, the earlier you act. Your vet may perform a vaginal swab to look for sperm, which is definitive proof that mating took place. However, the absence of sperm on a swab doesn’t rule out that it happened.
Be prepared to tell your vet approximately when the mating occurred, whether it was observed or suspected, and your dog’s current stage in her heat cycle. This information helps determine which treatment approach is safest and most likely to work. If you’re unsure whether mating actually happened, your vet can monitor for pregnancy with ultrasound starting around 25 days after the suspected event, giving you time to decide on next steps before the window for medical intervention closes.
Cost and Practical Considerations
Drug-based pregnancy termination in dogs is not as simple or cheap as picking up a pill at the pharmacy. Prostaglandin protocols require multiple vet visits or hospitalization, ultrasound monitoring, and careful dosing. Combination protocols may be shorter but still involve professional oversight. The progesterone blocker injection, where available, is the most straightforward medical option, but it isn’t licensed in every country, including the United States.
An emergency spay is a one-time cost that also eliminates any future reproductive concerns. For many dog owners dealing with an accidental mating, especially if they weren’t planning to breed their dog, it ends up being the most practical long-term choice. Your vet can walk you through the options that are available in your area and help you weigh the costs, risks, and your dog’s individual health factors.